The present invention relates to a system and method for removing gas from a stream of a mixture of gas and particulate solids. The invention is especially applicable for removing air from a stream of a mixture of pulverized coal and air to be injected to a pulverized coal furnace.
For use in pulverized coal furnaces, coal is typically pulverized in a mill into a particulate state and then delivered to the furnace, suspended in air. It is conventional to use the same air for grinding the coal, drying the coal, transporting the coal to a burner, and finally, for injecting the coal into the combustion chamber of the furnace. This air is commonly referred to as xe2x80x9cprimary airxe2x80x9d. The amount of primary air used for injecting the coal into the combustion chamber is an important variable with respect to the ignition and combustion efficiency of the coal. The amount of air is not, however, generally variable because of the requirements of adequate grinding, drying and transportation of the coal. Therefore, the ratio of primary air to coal resulting in optimal ignition and combustion efficiency is generally not achieved unless the amount of air is controlled separately, prior to the burner.
When burning low volatile fuels, such as anthracite, which are not easily ignitable, a decrease in the primary air-to-coal ratio may be required for efficient ignition and burning. Especially, there is a need to arrange means to remove excess primary air when the fuel is changed to a harder-to-burn coal. Also, low load burning may require a decrease in the amount of primary air injected into the combustion chamber to offset the decrease in fuel.
The idea of arranging a centrifugal separator above a vertical burner in order to control the concentration of fuel delivered to the burner is disclosed in U.S. Pat. No. 2,118,600. U.S. Pat. No. 4,412,496 shows an external cyclone to be used at low loads for separating a large quantity of air from a coal-air mixture, and producing a coal-rich stream to be delivered to the furnace through an inner nozzle of a burner and a coal-lean stream delivered to the furnace through an annular passage in a coaxial relationship to the inner nozzle. The cyclone separators are complex and expensive constructions which, especially when used with horizontal burners, may lead to problems related to the transport of the dense stream to the burner.
Also, many other solutions have been suggested for removing air from a coal-air stream. U.S. Pat. No. 4,497,263 discloses a burner with a louvered conical inner part to increase the coal concentration of the inner stream. U.S. Pat. No. 4,448,135 suggests a take-off conduit to be arranged downstream of an elbow section in a burner to remove a coal-lean portion from the coal-air stream. U.S. Pat. No. 5,090,339 shows a throat nozzle immediately upstream of a sleeve part, thus directing a coal-rich stream into the central section of a burner. These types of constructions may suffer high pressure losses and/or have a low separation efficiency.
Japanese patent publication No. 60-194208 discloses an arrangement in which a coal-air mixture is introduced tangentially into an annular space of a horizontal burner, whereby the coal-rich and coal-lean portions are divided by centrifugal force into radially separate partitions. The coal-lean portion is then directed in a switch-over part of the burner, located within the windbox of a burner system, through narrow channels from the inside of the coal-rich portion to outside of it, to be mixed with secondary air. In this construction, both portions of the coal-air mixture are layered in the same combustion zone. Moreover, when using this construction, the coal-rich stream may be disrupted by the channels of the coal-lean stream, and there is a considerable and non-controllable pressure loss of the coal-lean stream, which limits the amount of air which is separated from the coal-rich stream.
It is an object of the present invention to provide a system and method for removing gas from a stream of a mixture of gas and particulate solids.
Especially, it is an object of the present invention to provide a system and method for removing gas from a stream of a mixture of gas and particulate solids without causing stagnation of the solids or any major pressure drops in the stream.
Also, it is an object of the present invention to provide a system and method for a pulverized coal fired furnace in which the coal concentration of a coal-air stream to be injected into the furnace can be effectively increased.
Additionally, it is an object of the present invention to provide a system and method for a pulverized fuel fired furnace in which a controllable amount of excess air can be removed from a fuel-air stream, upstream of a burner, using a simple device.
In order to achieve these and other objects, the present invention provides a system for removing gas from a stream of a mixture of gas and particulate solids. The system comprises a separation vessel having an upstream end portion and a downstream cowl section, the upstream end portion having an inlet for introducing the stream tangentially into the vessel so as to separate centrifugally from the stream (i) a rich portion comprising a solids-rich mixture, proceeding along a helical path at an outer annular portion of the separation vessel to the cowl section, and (ii) a lean portion, comprising a solids-lean mixture, proceeding along a helical path at an inner portion of the separation vessel to the cowl section; multiple helical ports through which the rich portion proceeds axially through the cowl section, without significantly losing its momentum, to be discharged axially from the separation vessel; and multiple passages between the helical ports through which the lean portion proceeds radially outwards to be discharged from the separation vessel.
In another aspect, the present invention provides a method for removing gas from a stream of a mixture of gas and particulate solids, comprising the steps of introducing the stream tangentially into a separation vessel through an inlet at an upstream end portion of the vessel; separating centrifugally from the stream a rich portion, comprising a solids-rich mixture, and a lean portion, comprising a solids-lean mixture; allowing the rich portion to proceed along a helical path at an outer annular portion of the separation vessel to a cowl section at a downstream end portion of the separating vessel; allowing the lean portion to proceed along a helical path at an inner portion of the separation vessel to the cowl section; allowing the rich portion to proceed axially through the cowl section through at least two helical ports, without significantly losing its momentum, and discharging the rich portion axially from the separation vessel; allowing the lean portion to proceed radially outwards through multiple passages between the helical ports and discharging the lean portion from the separation vessel.
According to a typical application of the present invention, air is removed from a coal-air stream to be injected to a low volatile coal burner located on a wall of a pulverized coal fired furnace. Typically, a coal mill provides the original fuel-air stream, which is pneumatically transported to a set of burners. The burners are preferably arranged on the vertical sidewalls of the furnace, thus having a horizontal axis, but they may also be in another direction. According to a preferred embodiment of the present invention, upstream of each burner is connected a primary air separator. According to another preferred embodiment of the present invention, upstream of more than one burner is connected a common air separator.
One of the advantages of the air separation system according to the present invention is that it functions horizontally and can thus be connected to burners having a horizontal axis, without any constructional or operational difficulties. On the other hand, the air separation system according to the present invention is not designed as an integral part of a burner, but it can be located at some distance from the burner, typically outside the windbox of the burner system. Thus, the air separation system according to the present invention can be used in connection with different types of burners and suits well to retrofits of pulverized coal power plants.
The basic idea of the present invention is to maintain the momentum of the fuel-air mixture while it proceeds through the separator. By permitting the mixture to flow smoothly through a separation vessel, without reversing or suddenly changing the flow direction of the fuel-air stream, pressure losses can be simultaneously minimized and stagnation and settling of solids avoided.
It is generally known to introduce a mixture of gas and solid particles tangentially into a tubular separation vessel and to allow the centrifugal force to separate particles from the gas as the mixture proceeds through the vessel. However, in such systems, a particle-rich stream is formed at the outer annular portion of the vessel and a particle-lean stream is correspondingly formed at the inner portion. In order to remove excess gas from the particle-lean stream, the particle lean stream should be directed outwards and away from the particle-rich stream without significantly disrupting the smooth flow of the particle-rich stream.
According to the present invention, the removal of excess gas from a solids-gas stream, or a coal-lean stream from a coal-air mixture, takes place in a cowl section at the downstream end portion of the separation vessel. The fuel-rich portion flows axially across the cowl section through a set of helical ports, i.e., helically twisted short channels, and the coal-lean portion flows radially outwards through passages between the helical ports. Thus, the flow of the coal-rich stream is confined from a full annular space to multiple channels, or ports, but the ports are formed in a specific way to maintain the momentum of the stream. The coal-rich stream typically proceeds from the separation vessel to a burner on a sidewall of the furnace where it is mixed with secondary air and combusted.
According to a preferred embodiment of the present invention, the cowl section is defined by an outer wall and first and second end plates. The coal-rich and coal-lean portions of the coal-air mixture enter the cowl section through the corresponding inlet openings in the first end plate. The upstream ends of the helical ports are connected to the inlet openings of the coal-rich portion, and thus, the coal-rich portion is divided into multiple coal-rich substreams. The downstream ends of the helical ports are connected to the outlet openings for the coal-rich substreams in the second end plate, and the coal-rich substreams are discharged from the cowl section through the outlet openings. The coal-lean portion is discharged from the cowl section through an outlet opening in the outer wall.
According to a preferred embodiment of the present invention, a conical tube is provided within the cowl section, and radially inside of the conical tube, a cylindrical tube. The conical and cylindrical tubes form the roofs and floors, respectively, of the helical ports. The roof and the floor of each helical port are connected by two sidewalls, which may be referred to as leading and trailing sidewalls, as the ports are at an angle with respect to the axis of the helical trajectory of the stream.
The helical ports are twisted around the cylindrical surface so that the angular orientation of the outlet openings is rotated from that of the corresponding inlet openings. The direction and the extent of the rotation of the ports correspond to those of the trajectory of the coal-air stream around the axis of the separation vessel. The rotation angle varies, preferably, from about 20xc2x0 to about 40xc2x0, but it can be, e.g., from about 5xc2x0 to about 60xc2x0, depending on the axial length of the cowl section and the pitch of the trajectory of the coal-air stream.
The coal-lean portion enters the cowl section through a circular inlet opening arranged at the first end plate of the cowl section radially inside the inlet openings of the coal-rich substreams. The second end wall of the cowl section does not include outlet openings for the coal-lean portion. Instead, the conical and cylindrical tubes have elongated openings in between the helical ports, through which the fuel-lean stream can flow radially outwards. Typically, the helical ports and the first and second end plates define radial passages between the openings in the conical and cylindrical tubes, through which passages the fuel-lean portion can flow radially outwards.
According to the present invention, the coal-lean stream is allowed to maintain its rotational momentum while proceeding through the separation vessel, but the axial momentum is affected by the second end plate of the cowl section. As the lean stream mainly consists of gas, it will, however, change its direction without causing any major pressure drops or solids stagnation. The elongated openings in the cylindrical and conical tubes provide a wide region and enough cross-sectional area for the change of the flow direction.
According to a preferred embodiment of the present invention, the coal-lean substreams are collected in an annular collection space arranged between the conical tube and the outer wall of the cowl section. The fuel-lean portion may be discharged from the collection space via a tangential discharge channel connected to an opening in the outer wall. Preferably, the collection space includes block-off means, which forces all of the lean stream in the collection space to circulate in the same direction before being discharged via an outlet channel. In this way, all solids will be effectively swept off from the collection space. The fuel-lean portion discharged from the cowl section may be introduced to the furnace through a nozzle spaced away from the nozzle of the fuel-rich portion, or it may be conveyed back to the coal mill, or to some other remote location.
According to a preferred embodiment of the present invention, the cylindrical tube within the cowl section, including openings for the fuel-lean portion and forming the bottoms of the helical ports, extends to some extent upstream of the first end plate of the cowl section. The inlet openings for the coal-lean and coal-rich portions are located radially inside and outside, respectively, of the extension of the cylindrical tube. Thus, the extension of the cylindrical tube forms a separating wall which separates the rich and lean streams from each other as early as upstream of the cowl section.
In some applications, funnel means can be arranged on the radially outer side of the separating wall, upstream of the first end plate of the cowl section, directing the particle-rich stream into the inlet openings of the helical ports. Also, there can be inner vanes connected with the inner side of the cylindrical tube, or the separation wall, the vanes being curved like the helical trajectory of the coal-air stream. Preferably, the inner vanes extend from the upstream end of the separation wall to the second end plate of the cowl section. The inner vanes are preferably connected to the trailing sidewalls of the helical ports, adjacent to the leading edges of the radial passages between the helical ports conducting the lean flow smoothly to the openings between the helical ports.
In some applications of the present invention, an inner tube is provided around the axis of the separation vessel, and the fuel-air mixture flows in an annular volume around the inner tube. The inner tube may, e.g., include a heavy oil start-up burner. Preferably, the inner vanes extend radially inwards up to the inner tube, thus dividing the lean stream into several substreams. However, the present invention may also be used in connection with a separation vessel without an inner tube, whereby the air-fuel mixture fills the separation vessel all the way to the axis of the separation vessel.
The general features and advantages of the present invention are described above in connection with the treatment of primary air to be led to the burners of a pulverized coal fired furnace. It is, however, obvious to a skilled person in the art that the present invention also is well suited to other applications, e.g., in other processes such as in the chemical industry or bulk material handling, where separation of excess gas from a stream of a mixture of gas and particulate solids is required.